CN114423541A - Composite powder having iron-based particles coated with graphene material - Google Patents

Abstract

The present invention relates to graphene-coated iron-based particles and a method for producing the same. Composite powders suitable for powder metallurgy and additive manufacturing processes are provided comprising particles of an iron-based material with a coating of a graphene-based material, wherein the concentration of the graphene-based material is between 0.1 wt% and 1.0 wt%.

Description

Composite powder with iron-based particles coated with graphene material

technical field

The present invention relates to graphene-coated iron-based particles and methods for their manufacture, in particular to stainless steel and iron particles coated with graphene or graphene-based materials to optimize particles for use in additive manufacturing processes.

Background technique

Additive Manufacturing (AM), or 3D printing, is a manufacturing technique that allows complex 3D objects to be formed under computer control. It allows rapid prototyping and manufacturing of plastic and metal parts. Additive Manufacturing is an umbrella term that includes several technologies such as Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Electron Beam Melting (EBM), Fused Deposition Modeling (FDM) and Stereolithography (SLA) etc. technology.

Metal powder-based technologies dominate the AM field for the production of metal products. Final products with complex geometries and tailored properties such as strength and hardness can be fabricated by powder-based AM. Parts are manufactured by melting metal powder layer by layer, which is carried out by laser or electron beam heating. Typically, the layers are formed by a method commonly referred to as the powder bed method. In the powder bed method, a machine reads data from a 3D CAD model and lays down successive layers of powder metal. The layers are melted together using a computer-controlled electron or laser beam. The final part is built in this way. The process is carried out under vacuum (electron beam) or controlled atmosphere (laser beam), which makes it suitable for making parts from reactive materials with high affinity for oxygen, such as titanium and iron.

The distribution of metal powder is critical in the manufacturing process. Metal powder is typically supplied through nozzles to the build platform, or after the first layer, onto the top of the part under construction. Precision rakes are often used to level the supplied metal powder onto the surface of the top. Alternatively, the powder can be expanded to form a powder bed. Keeping the thickness of the bed as well as the density (packing density) constant within a given tolerance is a major concern for all techniques utilizing metal powders. A number of physical and chemical properties affect the "behavior" of a metal powder when forming a powder bed, including particle size and shape, surface roughness, and surface chemistry such as the propensity to react with surrounding materials. These properties are often summed up in density scales such as packed or tapped density and scales related to how the metal powder flows or "flows". As the technology progresses towards thinner layers to better control the build process and material properties, the need to control packing density and flow increases. Also, melting techniques for AM place different requirements on the starting powder and may have varying sensitivities to flow characteristics. For example, AM methods utilizing laser sintering/melting typically require smaller metal particle sizes than electron beam-based methods. In general, smaller particle sizes exacerbate flow problems.

Fill and liquidity are considered problem areas in the AM community. Said problems have been solved, for example, by controlling the environment (especially moisture), introducing coatings to make the particles inert, and by adding lubricants such as graphite-containing lubricants to the powder. However, the alloys that form the final product, such as stainless steel alloys, tend to be sensitive to impurities. For example, carbon content can significantly affect stainless steel properties, and only slight changes can be problematic. Therefore, any additive or composite material should either not affect the properties of the final product, or should be controlled in a way that the effect is controllable, reproducible and non-deteriorating.

For other techniques than AM, such as classical powder metallurgy, or PM, including the production of so-called green bodies and advanced sintering techniques such as hot isostatic pressing, HIP and wet binder techniques, better control Fill and flow are also important.

WO 2018/189146A1 discloses sliding contacts formed from Ag and graphene oxide composite materials, wherein Ag+GO composite powder is formed as an intermediate product. A GO content of about 0.01 wt% was found to be suitable for significantly reducing the friction of the final product, ie the sliding contact.

US 10,150,874 discloses steel and/or zinc cladding for corrosion inhibition, wherein the cladding contains graphene.

US 2011/0256014 discloses a graphene coating of "base metal powder". Graphene acts as a thin layer between metal particles. The graphene layer is formed by reduction of graphene oxide.

WO 2019/054931 discloses a multilayer graphene material that can be disposed on a substrate, such as a metal substrate. The multilayer graphene material comprises layers of graphene-based material, and between the graphene-based layers there is a third intermediate layer comprising a salt having ions comprising at least two cyclic planar groups, the The groups are capable of forming π-π stacking interactions with the layer comprising the graphene-based material.

In the prior art, there is still a need for composite metal powders with flow properties optimized for powder metallurgy and additive manufacturing.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a composite powder suitable for additive manufacturing and powder metallurgy, especially a composite powder comprising particles having an iron-based core and a graphene-based cladding layer.

This is achieved by the composite powder defined in claim 1 and the method defined in claim 10 .

The composite powder according to the invention is suitable for powder metallurgy and additive manufacturing processes and comprises iron-based material particles with a coating of graphene-based material, wherein the graphene-based material is present in a concentration between 0.1 wt% and 1.0 wt% .

According to aspects of the invention, the concentration of the graphene-based material is between 0.1 wt% and 0.95 wt%, more preferably between 0.1 wt% and 0.5 wt%.

According to one aspect of the present invention, the iron-based material of the particles comprises pure iron with unavoidable impurities.

According to one aspect of the invention, the iron-based particle material of the particles is stainless steel with unavoidable impurities.

According to one aspect of the present invention, the particle size distribution of the iron-based material particles is such that most of the particles are in the range of 1-500 μm, preferably in the range of 1-100 μm, more preferably in the range of 1-50 μm.

According to one aspect of the present invention, the graphene-based material of the cladding layer is graphene oxide (GO).

According to one aspect of the present invention, the graphene-based material of the coating layer is reduced graphene oxide (rGO).

According to one aspect of the present invention, the graphene-based material of the coating layer is a mixture of graphene oxide (GO) and reduced graphene oxide (rGO).

The method of the present invention comprises the following steps:

- providing iron-based metal powders with a known particle size distribution;

- providing graphene-based materials in dispersion;

- adding an alkaline substance to dilute the graphene-based material and adjust the pH, while recording the concentration of the graphene-based material in the solution, wherein the pH is adjusted to between 3 and 9;

- separating graphene agglomerates of said graphene material by sonication or stirring;

- dispersing the iron-based metal powder in deionized water or a water/alcohol mixture to produce a slurry having a predetermined iron-based metal to water weight ratio;

- adding the graphene material dispersion to the iron-based metal powder slurry at certain time intervals or at a predetermined rate, and mixing well for a predetermined period of time; and

- drying the composite powder,

The amount of the graphene material dispersion added therein is adjusted so that the concentration of the graphene material in the dried composite powder is between 0.1 wt % and 1.0 wt %.

According to one aspect of the present invention, wherein the amount of graphene material dispersion added is selected such that the concentration of said graphene material is between 0.1 wt% and 0.95 wt%, preferably between 0.1 wt% and 0.5 wt% .

According to one aspect of the present invention, the iron-based material of the particles comprises pure iron, and in the dilution and pH adjustment steps, the pH is adjusted within 4-8, preferably within 5-7.

According to one aspect of the present invention, the iron-based material is stainless steel, and in the dilution and pH adjustment steps, the pH is adjusted within 3-8, preferably within 4-7.

According to one aspect of the present invention, the graphene-based material is graphene oxide (GO).

According to one aspect of the present invention, the graphene-based material is reduced graphene oxide (rGO) or a mixture of reduced graphene oxide and graphene oxide.

Thanks to the present invention, composite powders with improved flow and fractal surfaces are provided, greatly improving powder handling in AM and other PM-based technologies.

One advantage is that the graphene material coating reduces oxidation of iron-based material particles.

In the following, the invention will be described in more detail by way of example, with regard to its non-limiting embodiments, with reference to the accompanying drawings.

Description of drawings

Fig. 1 is the schematic diagram of the method of the present invention;

Fig. 2a is a schematic diagram of a metal particle in the prior art, and Fig. 2b is a schematic diagram of a metal particle coated with a graphene material of the present invention;

Figure 3 is a diffractogram of various powders with and without GO coating produced using various pHs;

Figures 4a-b are SEM images of embodiments of the invention comprising stainless steel particles, c) is an SEM image showing unwanted agglomerations comprising stainless steel particles;

Figures 5a-b are SEM images of embodiments of the invention comprising pure iron particles;

6a-d are SEM images of composite powders containing pure iron metal particles and graphene oxide content of a) 0.05wt%, b) 0.1wt%, c) 0.2wt% and d) 0.5wt%, where b) represents Embodiments of the invention comprising pure iron particles;

Figures 7a-b are graphs showing avalanche angles, failure energies and avalanche energies at increasing concentrations of graphene material for embodiments of the invention comprising a) stainless steel particles and b) pure iron particles; and

Figures 8a-b are graphs showing surface fractals at increasing concentrations of graphene material for embodiments of the invention comprising a) stainless steel particles and b) pure iron particles.

Detailed ways

The following terms are defined and used throughout the specification and claims:

at% is an abbreviation for atomic percent, that is, the number of atoms of one kind compared to the total number of atoms;

wt% is an abbreviation for weight percent, i.e. the weight of one compound in a mixture or composite compared to the total weight of all compounds;

Graphene is an atomically thick planar sheet of carbon atoms arranged in a hexagonal lattice structure;

A graphene-based material is a layered material that contains at least 30 at% of carbon and has properties generally ascribed to graphene-like materials, the graphene-based material can be any type of graphene, such as single-layer graphene, few-layer graphene , multilayer graphene, graphene oxide (GO), reduced graphene oxide (rGO), and graphene nanosheets (GNP).

An iron-based powder material is a material with iron as the main component, such as but not limited to pure iron and stainless steel. The stainless steel may be, for example, grade 316 austenitic steel or equivalent. Typical particle sizes of powder materials suitable for AM and PM are in the range of 1-500 μm, depending on the AM/PM method used. For AM methods using laser melting/sintering and for conventional PM, particle sizes in the range of 1-100 μm are most suitable. A comprehensive review is "Powders for powder bed fusion: a review", Silvia Vock et al., Progress in Additive Manufacturing, https://doi.org/10.1007/ s40964-019-00078-6, incorporated herein by reference. The iron-based powder materials used as starting materials for the process of the present invention are commercially available in a wide range of compositions, particle size distributions and qualities. The starting materials can be produced, for example, by gas atomization or water atomization.

Flowability or powder flowability is defined as the ease with which a powder flows under a specified set of conditions. Some of these conditions include: the pressure on the powder, the humidity of the air surrounding the powder, and the equipment through which the powder flows or flows. Flowability can be measured by rotational powder analysis (RPA), which gives a set of parameters characterizing the flow properties of the powder material being analyzed. The properties include: avalanche angle [°], failure energy [KJ/Kg], avalanche energy [KJ/Kg], and surface fractal.

Avalanche Energy [kJ/kg] - The energy released by an avalanche. Calculation: The energy level of the powder after the avalanche minus the energy level before the avalanche. RPA reports the average avalanche energy for all powder avalanches.

Break energy [kJ/kg] - Calculation: The highest energy level of the sample powder before the onset of the avalanche minus the lowest energy level possible for the powder (flat and uniform surface). It is based on the volume and mass of the powder. This value represents the amount of energy required to initiate each avalanche.

Avalanche angle [°] - The angle of the powder at the maximum amount of powder before the start of the avalanche. The metric is the average of all avalanche angles. It is calculated from the center point of the powder edge to its vertex. This angle is the average angle required to initiate and maintain powder flow.

Surface Fractal - Surface fractal is the fractal dimension of the powder surface and provides an indication of the roughness of the surface. Measurements were taken after each avalanche to determine how the powder reorganized itself. If the powder forms a smooth and uniform surface, the surface fractal will be close to two. Rough and jagged surfaces produce surface fractals greater than five. For applications requiring uniform powder distribution, such as AM, the closer the surface fractal is to two, the better the powder will perform.

A method of preparing a metal powder suitable for AM comprising iron-based particles will be described with reference to FIG. 1 , the method comprising the following main steps:

- (not shown) to provide iron-based metal powder with known particle size distribution.

- (not shown) providing the graphene-based material as a dispersion.

-(a) Dilute the graphene-based material with distilled water or other diluent and adjust its pH, and adjust the pH by adding an alkaline substance such as NaOH (aqueous) until the pH is within a predetermined range. The concentration of the graphene-based material in the solution is recorded so that the final ratio between the graphene-based material and the iron-based material can be controlled,

-(b) separating the graphene agglomerates of the graphene material by, for example, sonication or thorough stirring.

-(c) Dispersing the iron-based metal powder in deionized water or other liquid to produce a slurry having a predetermined iron-based metal to water weight ratio.

-(d) adding the graphene material dispersion to the iron-based metal powder dispersion at time intervals or at a predetermined slow rate, selected so that mixing will be effective. The graphene material and the iron-based metal powder are thoroughly mixed for at least 2 hours. The amount of graphene material dispersion added was adjusted so that the concentration of graphene material in the final dry composite powder was between 0.1 wt % and 1.3 wt %.

-(e) drying the composite powder.

The method may optionally include one or both of the following steps to be taken prior to the drying step:

-(e2) filtering the composite powder

-(e3) Additional solvent washing of the filter cake (filtered composite powder) to remove any impurities such as free graphene or salts.

The filtering step should be considered as a non-limiting example. As will be appreciated by those in the art, filtration or separation can be performed in various ways using different known filtration or sieving techniques.

According to one embodiment of the present invention, the graphene material is graphene oxide (GO) in the form of a high concentration (about 2.5 wt%) graphene oxide paste or solution. The iron-based material is pure iron or stainless steel with a particle size distribution in the range of 1-100 μm, such as grade 316 austenitic steel or equivalent steel. According to the embodiment, the method comprises the steps of:

(A) Dilution and pH adjustment of graphene oxide slurry.

1. Transfer the specified amount of GO paste by effective mass into a container.

2. Add deionized water.

3. Check the pH of the diluted GO solution. Note: The initial pH of the solution is often around pH 2.

4. Adjust the pH of the solution to a range of 5 to 8 by adding NaOH 1M solution (pH 14) or equivalent. Adjustment to target pH is accomplished by addition of NaOH 0.1M solution or equivalent. For stainless steel materials, a pH in the range of 3-8 is suitable. For pure iron materials, a pH in the range of 4-8 is suitable due to increased oxidation at lower pH.

5. Weigh the mass of the solution and calculate the final concentration.

(B) Separate graphene agglomerates by sonicating the GO solution for at least 1 hour.

(C-D) Coated metal particles.

1. Weigh the target amount of metal powder.

2. Calculate the amount of GO solution required to coat the particles based on the target concentration.

3. Transfer the GO solution to an appropriate container and add deionized (DI) water in a 1:1 ratio.

4. Sonicate the solution for 1 hour at room temperature.

5. Transfer the metal powder to a rotary mixer such as a rotary evaporator and add deionized water until the powder is completely covered.

6. Mix the metal powder in the rotary mixer at 90 r.p.m for 15 minutes.

7. Add the prepared GO solution to the rotary mixer.

8. The powder was mixed with the GO solution in a facility rotary mixer at 90 r.p.m for 2 hours.

9. Activate the rotary evaporator vacuum pump, cooler and hot water bath to dry the solvent. Alternatively, transfer the mixture to a separate spin drying vessel.

a. Water bath temperature: 88℃

b. Speed: 90r.p.m

c. Vacuum 200 mbar – 100 mbar

d. Cooler temperature: 3℃-10℃

10. Once the powder is completely dry, turn off the rotary evaporator and remove the material from the container/balloon.

11. Grind the material to a fine powder without lumps.

12. Dry the powder in a vacuum oven at 88°C under high vacuum for 24 to 35 hours.

Embodiments of the method may optionally include one or a combination of the following steps to be taken prior to the drying step (step 9):

- Filter the coated powder using suction in a buchner funnel to remove most of the water

- Clean the filter cake in the Buchner funnel with deionized water (or ethanol) to remove free graphene and/or salts

- Place the filtered powder in an oven at 60°C (or place the powder in a flask and continue with step 9) to dry for at least 12 hours, then continue with step 11.

In the above examples, water was used as the process liquid. Other water-miscible solvents, such as alcohols such as ethanol, or mixtures of alcohols, can also be used. Mixtures of water and one or more alcohols, such as water/ethanol mixtures, are also embodiments of the method.

The experimental parameters, detailed time, pressure, solvent and temperature, given in the embodiment using GO, should be considered indicative. The exact parameters will depend on the equipment used, the amount of material used, and personal choice or preference with regard to, for example, processing time in relation to temperature. However, based on these indicative parameters, those skilled in the art will be able to make the necessary adjustments for specific equipment and other conditions.

As described in step (a) of the general method and steps 3-4 in the above embodiments, controlling and adjusting the pH is a way of controlling the formation of the coating. At lower pH (1-2), there is an attractive electrostatic force between GO and Fe particles, but insufficient repulsive force between GO sheets, resulting in agglomerates, which are disadvantageous when trying to obtain a uniform coating of. In most cases mixing occurs instead. Fe particles are also severely oxidized at low pH (1-2). When the pH was increased (3-4), less GO agglomerates were formed, and the corrosion of Fe particles was acceptable for some applications. At a certain point, (during the processing steps/time) not much oxidation has occurred and there are very few agglomerates, but there is still an attractive electrostatic force between the GO sheet and the Fe particles. This is in the pH 5-9(10) region.

Increasing pH would also generate more negatively charged groups on the basal plane of GO sheets, which would be beneficial to obtain a good coating. However, at too high pH, the net surface charge of Fe particles also becomes negative, creating electrostatic repulsion between GO sheets and Fe particles, which can be clearly seen at pH values above 10, but pH above 7 may affect the quality of the coating. If the iron-based material itself has good corrosion resistance, such as stainless steel grades, such as grade 316, a lower pH can be chosen without risking oxidation of the particle surface. The effect of pH is summarized in Table 1.

pH powder coating Oxidation during processing 1 No (mixing occurs) high 2 No (mixing occurs) high 3 Yes acceptable 4 Yes acceptable 5 Yes none 6 Yes none 7 Yes none 8 Yes none 9 Yes none 10 Yes (to a lesser extent) none 11 yes (low level) none 12 no none 13 no none

Table 1: Effect of pH on coating formation and oxidation of pure iron particles.

According to one embodiment of the present invention, the pH is adjusted within 3-9, preferably within 3-7.

According to one embodiment of the present invention, the pH is adjusted within 5-8.

According to one embodiment, the iron-based material is pure iron and the pH is adjusted within 4-8, preferably within 5-7.

According to one embodiment, the iron-based material is stainless steel and the pH is adjusted within 3-8, preferably within 4-7.

Figure 3 is a diffractogram of various powders with and without GO coating produced from various pHs used. Here it can be observed that iron is slightly oxidized at pH ₃ (magnetite Fe3O4 peak can be seen ₎ , but still acceptable for some applications. For other pHs, no such oxidation was seen. Also, in the already coated powder, there are no peaks in the regions where GO agglomerates would appear in the diffractogram. This is an indication (at low values) that there is no free and agglomerated GO around the particles. This was also confirmed by SEM, where agglomerates of free GO were barely visible (scarlessly).

In one embodiment of the present invention, the graphene-based material is reduced graphene oxide (rGO), partially reduced graphene oxide, or a mixture of graphene oxide and reduced graphene oxide.

It should be noted that graphene oxide may be affected by the method described. For example, if the starting material is graphene oxide (GO), certain steps, especially the final drying step, can cause the reduction of graphene oxide, so that the final composite powder can also contain reduced graphene oxide (rGO). The reduction mechanisms of GO and how to control them are well known to the skilled person.

According to one embodiment, the metal particles are pure iron.

The method of the present invention produces a composite powder comprising iron-based metal particles with a graphene coating. The method can fine-tune the degree of coating and optimize the flowability of the composite powder by varying the concentration of graphene material in the process and thus also in the final composite powder.

Figure 2 schematically depicts a) two uncoated iron-based particles 20 of a prior art metal powder and b) two iron-based particles 21 coated with graphene material 22 forming a composite powder of the present invention . Metal-to-metal contacts of prior art metal powders generally result in much higher frictional forces than graphene-graphene contacts of composite powders of the present invention. This is shown by the enlarged portion of FIG. 2 . Metal-graphene contacts exhibit significantly lower friction than metal-metal contacts even when the particles are only partially covered by graphene material.

The SEM images of Figure 4a–c depict the stainless steel particles with graphene oxide coating of the composite powder. Figure 4a depicts stainless steel particles with a graphene oxide coating in a composite powder with a graphene oxide content of 0.2 wt%, verifying that the method of the present invention can produce coated iron-based metal particles. This was verified by morphological examination and EDS analysis.

The SEM image of Figure 4b shows the composite powder with a graphene oxide content of 0.5 wt% and shows that the composite powder is well dispersed. This was verified by morphological examination and EDS analysis.

As shown in the SEM image of Fig. 4c, increasing the graphene material concentration to or above 1.3 wt% will result in some agglomeration of particles in the composite powder.

The SEM images of Figures 5a and 5b show pure iron particles with a graphene oxide coating of the composite powder with a graphene oxide content of 0.1 wt%.

Figures 6a-d are SEM images of composite powders containing pure iron metal particles and graphene oxide content of a) 0.05 wt%, b) 0.1 wt%, c) 0.2 wt% and d) 0.5 wt%. Similar to the composite powders containing stainless steel particles, lower graphene oxide concentrations (0.05 wt% and 0.1 wt%) resulted in the presence of graphene oxide partially covering the particle surfaces. A graphene oxide concentration of 0.2 wt% resulted in complete coverage of the particle surfaces with graphene oxide. Further increasing the graphene oxide concentration (0.5 wt%) resulted in the separation of the excess graphene sheet agglomerates from the fully covered iron particles.

The flow properties were measured using rotational powder analysis (RPA), while the parameters avalanche angle [°], failure energy [KJ/Kg], avalanche energy [KJ/Kg] and surface fractal of stainless steel samples are shown in Table 2a (stainless steel) and Table 2. 2b (pure iron, with avalanche angle, failure energy, avalanche energy shown in the graphs of Figures 7a (stainless steel) and 7b (pure iron), reference sample (uncoated) and increasing concentrations left to right), Figures 8a (stainless steel) and 8b (pure iron) are surface fractals.

concentration reference 0,05wt% 0,1wt% 0,2wt% 0,5wt% 0,95wt% 1,3wt% Avalanche angle [°] 63,10 67,03 51,23 57,70 59,83 60,20 61,10 Destructive energy[KJ/Kg] 103,98 104,45 77,34 81,83 93,60 96,80 94,33 Avalanche energy [KJ/Kg] 40,24 37,14 32,30 25,35 35,76 41,33 35,80 fractal surface 6,64 4,74 2,81 3,11 3,46 3,33 3,56

Table 2a: Avalanche angle, failure energy, avalanche energy and fractal surface of composite powders with stainless steel particles

concentration reference 0,05wt% 0,1wt% 0,2wt% 0,5wt% 1,0wt% Avalanche angle [°] 55,67 53,8 56,1 56,2 57,2 60,5 Destructive energy[KJ/Kg] 97,99 80,4 77,8 81,83 80,5 79,6 Avalanche energy [KJ/Kg] 16,37 23,3 15,9 15 20 18,3 fractal surface 4.31 3,42 3,16 3,14 3,34 3,42

Table 2b: Avalanche angles, failure energies, avalanche energies and fractal surfaces of composite powders with pure iron particles

It is evident from the flowability measurements that significant reductions in parameters related to flowability and surface fractal are also evident for pure iron particles.

The composite powder of the present invention comprises particles having a core of iron-based material and a coating layer of graphene-based material, wherein the concentration of graphene-based material is in the range of 0.1 wt% and 1.0 wt%, preferably 0.1 wt% and 0.5 wt% between, more preferably between 0.1 wt% and 0.3 wt%. It is obvious to the skilled person that the optimum concentration range can be adjusted depending on the parameters of the iron-based particles, such as their particle size distribution, wherein the ratio of the surface area to the particle mass can be taken into account. With the knowledge that optimal ranges exist, basic geometric relationships, and the data presented here, such adjustments do not place an undue burden on the skilled person. The methods described above represent the preferred methods for producing the composite powders of the present invention.

By comparing the flow data (Tables 1a and 1b / Figures 7-8) with the SEM images, it can be noted that the positive effect on flow will be at graphene material concentrations that do not necessarily result in fully encapsulated metal particles, such as It started to appear at 0.1 wt%. The positive flowability effect appears to develop sufficiently at about 0.2 wt% to produce fully coated metal particles. As the skilled person will appreciate, terms describing the degree of coating of metal particles should be interpreted in a statistical sense: the composite powder will contain a mixture of fully coated and partially coated particles at all concentrations, "completely coated" "Coated metal particles" and "partially coated metal particles" are descriptions of representative composite particles at various concentrations.

According to one embodiment, the graphene-based material of the cladding layer comprises graphene oxide. As a result of the production method or as a result of further processing, the graphene oxide may have been at least partially reduced such that the cladding layer comprises a mixture of graphene oxide (GO) and reduced graphene oxide (rGO).

According to one embodiment of the present invention, the iron-based core of the composite powder has a particle size distribution in the range of 1-100 μm, a particle size range known to be suitable for laser sintering/melting and conventional PM. According to one embodiment, the particle size distribution of the iron-based core of the composite powder is in the range of 1-100 μm.

Both iron-based materials and graphene-based materials may contain inevitable impurities associated with the respective materials.

Experiment Details

Effects of pH:

To investigate the effect of pH on the coating process, a series of experiments from pH 1 to 13 were carried out. Solutions at pH 1 to 13 were prepared by adding NaOH to deionized water for samples above pH 6 or HCl to deionized water for samples below pH 6. The pH of each sample was controlled with a calibrated VWR pHenomenal 1100H pH meter. For samples at pH 6, use only deionized water as it is slightly acidic due to dissolved atmospheric carbon dioxide (CO2). To avoid changing the surface charge of graphene oxide (GO), resulting in changes in the salt concentration in each sample, no further salt concentration was intentionally increased. For each sample, 0.010 g of GO was diluted in 8 ml of the solution at the target pH and sonicated for 1 h. Then, 1 g of iron powder was added, followed by mixing for 1 minute. Samples were visually inspected before iron addition, 1 minute after mixing, and 1 hour after mixing. In addition to this, some powders were removed at 1 minute, 1 hour and 20 hours after mixing and left to dry at room temperature. Pure iron powder was also mixed at pH 3, 5 or 8 for 4 hours to analyze the effect of corrosion.

GO was diluted in deionized water and NaOH solution to obtain three dispersions with equal concentrations of GO at pH 3.0, 5.4 and 8.0. The dispersion was then sonicated for 60 minutes to dissolve any visible precipitate. Metal powder (5 g) and 10 g deionized water were added to a beaker to form a slurry. The sonicated GO dispersion was slowly added to the metal powder slurry with stirring and then mixed for a further 2.5 hours in a rotary evaporator (Büchi R-300) at 90 rpm (300 mbar pressure). The composite powder was filtered, rinsed with deionized water and dried at 50°C.

Stainless steel composition:

The stainless steel is an austenitic stainless steel with a composition of C 0.03%, Cr 17.0%, Ni 12.0%, Mo 2.5%, Si 0.7%, Mn 1.5%, S 0.03%, P 0.04% and the balance Fe.

Metal particle size distribution:

Typical particle size distributions of the stainless steel particles are given in Table 2.

Particle size (μm) D₁₀% 4.5 D₅₀% 10.5 D₉₀% twenty two

Table 2: Typical particle size distribution of grade 316 stainless steel powders

The pure iron particles contained Alfa Aesar 99.5% iron with a particle size distribution of about 10 μm.

Practical tests have been carried out with composite powders containing iron-based materials to manufacture objects with AM (SLM) as well as sintering. The composite powder works well in AM equipment, and the adjustment of printing parameters is considered to be no problem for skilled operators. The resulting objects have expected material properties compared to objects produced from the uncoated starting powder material.

Claims (18)

1. A composite powder suitable for powder metallurgy and additive manufacturing processes, the composite powder comprising particles having a core of an iron-based material and a coating of a graphene-based material, characterized in that the concentration of the graphene-based material is between 0.1 and 1.0 wt%.

2. The composite powder according to claim 1, wherein the concentration of the graphene-based material is between 0.1 wt% and 0.95 wt%, and more preferably between 0.1 wt% and 0.5 wt%.

3. The composite powder according to claim 1, wherein the iron-based material of the particles is pure iron.

4. The composite powder according to claim 1, wherein the iron-based particle material of the particles is stainless steel.

5. A composite powder according to any one of the preceding claims, wherein the core of iron-based material has a particle size distribution in which the majority of the particles is in the range of 1-100 μ ι η.

6. The composite powder according to claim 5, wherein the core of iron-based material has a particle size distribution in which the majority of the particles are in the range of 1-50 μm.

7. The composite powder according to any one of the preceding claims, wherein the graphene-based material of the coating layer is Graphene Oxide (GO).

8. The composite powder according to any one of claims 1 to 6, wherein the graphene-based material of the cladding layer is reduced graphene oxide (rGO).

9. The composite powder according to any one of claims 1 to 6, wherein the graphene-based material of the cladding layer is a mixture of Graphene Oxide (GO) and reduced graphene oxide (rGO).

10. A method of producing a composite powder suitable for powder metallurgy and additive manufacturing processes, the composite powder comprising particles of an iron-based material with a coating of a graphene-based material, the method comprising the steps of:

-providing an iron-based metal powder having a known particle size distribution;

-providing a graphene-based material in a dispersion;

-diluting the graphene-based material and adjusting the pH by adding a basic substance while recording the concentration of the graphene-based material in the solution, wherein the pH is adjusted to between 3 and 9;

-separating graphene agglomerates of the graphene material by sonication or stirring;

-dispersing the iron-based metal powder in deionized water to produce a slurry having a predetermined iron-based metal to water weight ratio;

-adding the graphene material dispersion to the iron-based metal powder slurry at intervals or at a predetermined rate and thoroughly mixing for a predetermined period of time; and

-drying the composite powder,

wherein the amount of graphene material dispersion added is adjusted such that the concentration of the graphene material in the dried composite powder is between 0.1 wt% and 1.0 wt%.

11. The method of claim 10, wherein the amount of graphene material dispersion added is selected such that the concentration of the graphene material is between 0.1 wt% and 0.95 wt%.

12. The method of claim 11, wherein the amount of graphene material dispersion added is selected such that the concentration of the graphene material is between 0.1 wt% and 0.5 wt%.

13. The method according to any one of claims 10 to 12, wherein the iron-based material of the particles comprises pure iron, and in the diluting and pH adjusting step the pH is adjusted to within 4-8, and preferably within 5-7.

14. The method according to any one of claims 10 to 12, wherein the ferrous material is stainless steel and in the diluting and pH adjusting step the pH is adjusted to within 3-8, and preferably within 4-7.

15. The method according to any one of claims 10 to 12, wherein the iron-based material of the particles comprises pure iron.

16. A method according to any one of claims 10 to 12, wherein the iron-based particle material of the particles is stainless steel.

17. The method of any one of claims 10 to 16, wherein the graphene-based material comprises Graphene Oxide (GO).

18. The method of any one of claims 10 to 17, wherein the graphene-based material comprises reduced graphene oxide (rGO).

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